1. Field of the Invention
The present invention relates to a write-once optical recording medium capable of achieving a high recording density.
2. Description of the Related Art
With spread of personal computers in recent years, importance of media for storing digital data is being enhanced. Recently, a recording medium capable of recording-reproducing digital data has come to be mounted to home electric appliances such as a digital video appliance in addition to a digital audio appliance. A recording medium for storing digital data has also come to be used in mobile appliances such as cellular phones. A disk-type medium, i.e., a recording disk, is widely used as such a recording medium because it permits easily performing recording-reproducing of a large amount of digital data, it is capable of achieving a high random access performance, and it is inexpensive.
The recording disk includes a magnetic disk and an optical disk. The magnetic disk is a magnetic recording type medium represented by a hard disk. The optical disk is an optical recording type medium represented by CD-R, CD-RW, DVD-R and DVD-RW.
The optical disk is based on CD standards (such as red book) and DVD standards (such as DVD book) and, thus, is excellent in its compatibility and in its removability. To be more specific, it is possible to carry an optical disk having information written with a certain drive, and to write and read information in and out of the optical disk with another drive.
The optical disk includes a read-only type (ROM) such as CD-DA, CD-ROM, DVD-VIDEO and DVD-ROM, a write-once type, in which information can be written only once, such as CD-R and DVD-R, and a rewritable type in which information can be written many times such as CD-RW and DVD-RW. As the optical disk capable of recording, the write-once type optical disk such as CD-R and DVD-R employing an organic dye in the recording layer, is most widely used because the manufacturing cost of the disk is low and, thus, the disk of this type is inexpensive. It should also be noted that, where the data capacity is as large as 700 megabytes or more, it is substantially unnecessary to erase data in preparation for the writing of new data. Thus, it practically suffices for the recording medium to be capable of writing information only once. The particular situation also provides the reason for the spread of CD-R and DVD-R.
In the write-once optical disk employing an organic dye, a recording region defined by grooves is irradiated with a laser beam. As a result, the resin substrate is heated to a temperature not lower than the glass transition point Tg of the resin substrate. At the same time, a film of the organic dye within the groove performs a photochemical reaction so as to set up a negative pressure. It follows that the resin substrate is deformed within the groove. The deformation of the resin substrate is utilized for forming a recording mark.
However, two serious problems remain unsolved in the write-once type optical disk. One of the problems is that a limit of high-speed recording is being reached in the DVD-R drive that is available nowadays. Presently, the highest recording rate in the DVD-R drive is four-times speed (4×), i.e., 3.49 m/sec×4=13.96 m/sec. In the DVD-R disk for high-speed recording, the groove is formed shallow, and the film of the organic dye is designed thin so as to achieve a high sensitivity. However, the shallow groove implies that the deformation amount of the substrate is small. Therefore, if the groove is formed shallower for improving the sensitivity, it is impossible that the recorded marks produce a sufficiently high phase difference, leading to a low degree of the signal modulation. It follows that read error for the data tends to be generated, resulting in failure to satisfy the DVD book standards. It should also be noted that the degree of the signal modulation of the land pre-pit arranged as address data is also lowered. It follows that the read error for the address data tends to be generated, resulting in failure to satisfy the DVD book standards.
The other problem to be noted is that a limit is being reached in data recording density for the DVD-R available nowadays. In order to improve further the recording density, it is necessary to decrease the size of the recording mark itself. However, since the shortest wavelength of the semiconductor laser is about 400 nm and the maximum numerical aperture of the objective lens is 0.65 to 0.85 nowadays, a limit is being reached in the spot diameter itself of the laser beam. It is very difficult to record a small mark with a high accuracy and to reproduce the recorded small mark by using a relatively large light spot.
As described above, it was difficult to satisfy both the higher recording speed and the higher recording density simultaneously in the conventional write-once optical recording medium such as a DVD-R.